MFC (conceptually) r328106: libfdt: Update to 1.4.6, use libfdt for overlays

This is a direct commit to stable/11 due to path changes in HEAD. Diffs have
been applied on top of their previous locations, with one minor adjustment
as per the "MFC" note below: a <stdlib.h> include in libfdt_env.h h as been
wrapped in an `#ifndef _STANDALONE` block to prevent build failures when
built in the bootloader context. These issues are not present in HEAD, thus
this differs from HEAD.

libfdt highlights since 1.4.3:

- fdt_property_placeholder added to create a property without specifying its
value at creation time
- stringlist helper functions added to libfdt
- Improved overlay support
- Various internal cleanup

Also switch stand/fdt over to using libfdt for overlay support with this
update. Our current overlay implementation works only for limited use cases
with overlays generated only by some specific versions of our dtc(1). Swap
it out for the libfdt implementation, which supports any properly generated
overlay being applied to a properly generated base.

This will be followed up fairly soon with an update to dtc(1) in tree to
properly generate overlays.

MFC note: the <stdlib.h> include this update introduces in libfdt_env.h is
apparently not necessary in the context we use this in. It's not immediately
clear to me the motivation for it being introduced, but it came in with
overlay support. I've left it in for the sake of accuracy and because it's
not harmful here on HEAD, but MFC'ing this to stable/11 will require
wrapping the #include in an `#ifndef _STANDALONE` block or else it will
cause build failures.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Update to latest git version of dtc to get new dtsv2 support,
including the include directive.

Fix minor build issue corrected by converting yypush_buffer_state and
yypop_buffer_state to yy_set_buffer_state and a hard-coded 100-deep
stack. It was easier to fix it here than to import that support into
our flex.

The new tools and test hardness remain unsupported at the moment.

Add missing files in f807af192828222dee7a5c9f94d999673bb4d8a1 import.